In the Microsoft Windows ecosystem, Dynamic Link Libraries (DLLs) are Portable Executable (PE) files that export functions callable by other programs. Unlike regular executables, DLLs have a special entry point function called DllMain, which is invoked automatically by the system loader when the DLL is loaded or unloaded, or when threads are created or destroyed. This entry point receives parameters indicating the reason for the call (e.g., process attach or detach). While DllMain is optional, it is commonly used to initialize resources or environment variables. However, threat actors can abuse this entry point to embed malicious code that executes immediately upon DLL load, without requiring explicit calls to exported functions. This technique can evade detection because many analysts focus on exported functions and overlook the DllMain code. Tools like regsvr32.exe and rundll32.exe, which load and execute DLLs, are common vectors for such attacks. A proof-of-concept DLL demonstrates that malicious code (e.g., spawning calc.exe) can run from DllMain upon loading, even if no exported function is invoked. This stealthy execution method is attractive to malware authors aiming to remain under the radar. The threat does not currently have known exploits in the wild but represents a medium severity risk due to its potential for covert execution and evasion. Detection requires awareness of the DllMain entry point and monitoring of DLL loading behaviors. The threat is documented by SANS ISC and highlights the need for reverse engineers and defenders to inspect DLL entry points, not just exported functions, during analysis.

European organizations running Windows environments are at risk of stealthy malware infections that exploit the DLL entry point to execute malicious code upon DLL load. This can lead to unauthorized code execution, persistence, and potential lateral movement within networks. The covert nature of this technique complicates detection and incident response, increasing the risk of prolonged undetected compromise. Critical sectors such as finance, healthcare, energy, and government, which rely heavily on Windows systems and legacy applications, may face increased exposure. The use of legitimate Windows tools like regsvr32 and rundll32 to load malicious DLLs can bypass traditional security controls, enabling attackers to blend in with normal system activity. This threat could facilitate espionage, data theft, or disruption of services if leveraged by advanced persistent threat (APT) groups. The medium severity reflects the balance between the technical complexity required to craft such DLLs and the significant impact stealthy execution can have on confidentiality and integrity of systems.

1. Enhance malware analysis procedures to include inspection of DLL entry points (DllMain) in addition to exported functions, ensuring no malicious code is hidden in the entry point. 2. Implement strict application whitelisting and code signing policies to allow only trusted DLLs to be loaded by critical processes. 3. Monitor and restrict the use of regsvr32.exe and rundll32.exe, especially blocking their execution with untrusted or unknown DLLs, using endpoint detection and response (EDR) tools. 4. Deploy behavioral detection rules that alert on anomalous DLL loading patterns, such as unexpected process creation from DLL entry points or unusual use of system utilities to load DLLs. 5. Conduct regular threat hunting exercises focusing on DLL load events and suspicious process creations linked to DLL entry point execution. 6. Educate security analysts and reverse engineers about the importance of examining DllMain during static and dynamic analysis of suspicious DLLs. 7. Keep Windows systems and security tools updated to leverage latest detection capabilities for DLL-based attacks. 8. Use sandboxing and automated analysis environments that execute DLLs fully, capturing any code run from entry points to detect hidden malicious behavior.